Coastal Lagoons and Estuaries of the Gulf of Mexico: Toward the Establishment of Ecological Indicators
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COASTAL LAGOONS AND ESTUARIES OF THE GULF OF MEXICO: TOWARD THE ESTABLISHMENT OF ECOLOGICAL INDICATORS Francisco Contreras E. and Ofelia Castañeda L. Coastal lagoons and similar ecosystems are characterized by a continuously changing environment as a result of the hydrological effects caused by the encounter of two water masses of different origin and physico-chemical properties, which in principle limits biodiversity. On the other hand and from another perspective, this encounter results in the presence of various habitats that allow the establishment of organisms, populations and even communities with different requirements. From a spatial point of view, the presence of areas with permanent freshwater influence favors the colonization of organisms originated from continental waters, whereas the constant tidal effect supplies species of marine origin. It is also necessary to emphasize the almost permanent presence of extensive areas where brackish conditions of 10 to 25 ppt present a suitable hydrological environment for the development of typically estuarine organisms and/or those efficiently adapted to it. Thus the average number of fish species per lagoon ranges from 50 to 100, molluscs from 50 to 90, and crustaceans from 40 to 70. Most lagoons are also closely connected to mangrove forests, which represent particularly rich habitats for birds (1,038 species of birds from 86 families are identified in Mexico), reptiles and mammals. In addition to this, the socio-economic importance of fishing activities carried out in coastal lagoons, particularly in the Gulf of Mexico, is unquestionable. Based on official statistics and on an analysis of the period from 1988 to 1999, it is possible to establish that the majority of fishing in the Mexican Gulf states is represented by river fishing, relative to the total volume of fishing (including deep-sea fishing), and maintains first place for 19 commercial species. All of these species are captured inside the lagoons or in very nearby areas. The abundance of some species is due to the presence of coastal ecosystems. The proportion of river to deep-sea fishing in terms of percentages is: Tamaulipas 91.2, Veracruz 80.4, Tabasco 91.1, Campeche 83.4 and Yucatán 85%. In economic terms, river species are commonly preferred over ocean species (shrimp, lobster, crayfish, etc.) and involve a considerable number of people and related activities. In summary, the coastal lagoons and estuaries constitute a hydrological effect resulting from the encounter of two types of water and represent the best indicator of the quality of the basin. Thus, knowledge of the main physico-chemical parameters offers an invaluable key to the management of coastal ecosystems and basins. The generation of databases regarding the characteristics of our aquatic ecosystems, both abiotic and biotic, becomes essential for the establishment of indicators for monitoring and control of the health of these systems. In the present work the results of a considerable number of years of sampling in several lagoons of the Gulf of Mexico are presented in an integrated form for the first time. This allows the establishment of some "normal" values for some abiotic parameters. Unpublished data from several authors are presented below (the sources are specified wherever applicable). A summary of hydrological data is presented with a brief description of each lagoon system, with the objective of collaborating with other investigators. 230 GENERAL CHARACTERISTICS The Mexican coastline of the Gulf of Mexico reaches from the states of Tamaulipas to Yucatán. Its total extension is 1,910 km and coastal lagoons cover a surface area of approximately 5,767 km2. The rivers with highest volume (Grijalva-Usumacinta, Papaloapan) and the largest estuarine ecosystems in the country (Laguna Madre and Laguna de Términos, covering 200,000 and 196,000 hectares, respectively) are located along this coastline. Several economic activities are carried out on the coast, among which river fishing, tourism, ranching, agriculture and oil extraction stand out. The geological origin of the coast of the Gulf of Mexico is responsible for the presence of extensive coastal plains and continental shelves, which reach from 170 to 269 km in the Campeche Bay and the Yucatán Shelf. Except for the northern portion of Tamaulipas and part of the Yucatán Peninsula, evaporation and precipitation are similar, in addition to the important freshwater contributions from numerous rivers, which promote a tendency to estuarine characteristics of the water bodies and oligohaline conditions in some cases, such as the Centla marshes in Tabasco. SOME BIOLOGICAL CHARACTERISTICS OF THE LAGOONS AND ESTUARIES OF THE GULF OF MEXICO From the biological point of view, the Gulf's coastal lagoons (Figure 12.1) present individual characteristics and diversity promoted by hydrological factors and by the rate of water exchange, as well as by the dominant influence, whether it is marine or fresh water, as determined by seasonality (Nixon 1981; Knoppers et al. 1991; Knoppers and Kjerve 1999; Alber 2002). However, there are some components that are shared, such as the presence of mangrove forests composed of well-known species (Rhizophora mangle, Laguncularia racemosa, Avicennia germinans and Conocarpus erectus), although the four species do not always coexist. Seagrasses are also strongly represented in the majority of coastal environments, especially towards the Yucatán. The phytoplankton is dominated by diatoms, and copepods are predominant in the zooplankton. Acartia tonsa is a frequent species in all of them; similarly, the presence of fish such as Mugil cephalus and Mugil curema is common. The number of crustacean, mollusc and fish species varies from one lagoon to another, and frequently this is due to particular characteristics of the environment, although in certain occasions it can be explained by the sampling method used by the researcher. Table 12.1 presents a summarized inventory of the Gulf of Mexico lagoons. LAGUNA MADRE This is the largest coastal lagoon in Mexico with a surface area of 200,000 ha. In high salinity areas the surrounding vegetation is dominated by the Suaeda nigra-Salicornia ambigua association; the latter is frequently replaced by Batis maritima and, on occasions, by a halophyte cover of Distichlis spicata and Monanthochloe littoralis. On the slightly better drained plains Spartina spartinae and Spartina densiflora settle over such associations. Ichthyoplankton is represented by 17 families, 25 genera and 29 species. The most abundant are the families Engraulidae, Gobiidae and Sparidae (Ocaña and Sánchez 1991). The caridea are represented by 231 Figure 12.1. Main coastal lagoons of the Gulf of Mexico. 232 Table 12.1. Summarized inventory of lagoons in the Gulf of Mexico. W. W Ha N. Lat. Long. Ha N. Lat. Long. Veracruz Tamaulipas Lag. Pueblo Viejo 9,300 22° 13' 97° 57' Lag. Madre 200,000 25° 48' 97° 54' 22° 05' 97° 50' Lag. El Catán 24° 01' 97° 23' Lag. Tamiahua 88,000 22° 06' 97° 46' Lag. Morales 3,200 23° 46' 97° 47' 21° 15' 97° 23' Lag. San Andrés 9,700 22° 47' 97° 54' Lag. Tampamachoco 1,500 21° 02' 97° 22' 22° 32' 97° 41' 20° 18' 97° 19' Lag. El Chairel 4,200 22° 10' 97° 51' Estuario Tecolutla 20° 29' 97° 04' Est. Río Pánuco 22° 28' 98° 38' a 20° 27' 97° 00' Total # of ecosystems 19 Lag. Casitas-Nautla 20° 15' 97° 00' 20° 06' 96° 00' Tabasco Lag. Grande 2,250 20° 06 96° 41' Lag. El Carmen 8,800 18° 18' 93° 53' 20° 02' 96° 38' Lag. La Machona 6,500 18° 14' 93° 45' Lag. San Agustín 172 19° 56' 96° 35' Lag. Tupilco 800 18° 26' 93° 27' 19° 55' 96° 30' 18° 24' 93° 25' Lag. Verde 110 19° 43' 96° 25' Lag. Mecoacán 5,200 18° 28' 93° 14' Lag. El Llano 2,360 19° 36' 96° 21' 18° 16' 93° 04' Lag. El Farallón 800 19°38' 96° 24' Pantanos de Centla 300,000 18° 39' 92° 47' Lag. La Mancha 192 19° 42' 96° 32' 17° 57' 92° 06' a 19° 34' 96° 27' Total # of ecosystems 37 Est. La Antigua 270 19°29' 96° 18' Lag. Mandinga 3,250 19° 06' 96° 06' Campeche 19° 00' 96° 02' Lag. Atasta 3,600 18° 40' 92° 14' Lag. Alvarado 6,200 18° 59' 95° 57' Lag. Pom 5,200 18° 33' 92° 01' Lag. Camaronera 3,900 18° 43' 95° 42' Lag. deTérminos 170,000 19° 00' 92° 00' Lag. Sontecomapan 891 18° 34' 95° 04' 18° 20' 91° 10' a 18° 30' 95° 00' Total # of ecosystems 12 Lag. del Ostión 1,270 18° 15' 94° 42' 18° 07' 94° 37' Yucatán Est. Río Coatzacoalcos 900 18° 10' 94° 25' Lag. Celestún 3,100 20° 59' 90° 28' Est. Río Tonalá 18° 14' 94° 08' 20° 46' 90° 19' a Total # of ecosystems 101 E. Yucalpetén 21° 19' 89° 47' Lag. Chelém 21° 15' 89° 40' Dzilam de Bravo 400 21° 22' 88° 59' Ría Lagartos 10,000 21° 34' 88° 15' 21° 32' 87° 35' a Total # of ecosystems 5 TOTAL GULF ECOSYSTEMS: 174 a The “total # of ecosystems” includes the total number of associated lagoons and equivalent water bodies 233 six genera and nine species, with predominance of Hippolyte pleurocantharus and Tozeuma carolinense (Barba et al. 1993, in Castañeda and Contreras 2001). Thirty-eight species of molluscs were identified. The dominant faunistic group in the lagoon is composed by the pelecypods Mulinia lateralis, Anomalocardia auberiana and the gastropod Acteon punctostriatus, which are very abundant (García-Cubas 1978). Seventy-eight species of fish are reported, of which the following stand out: Lagodon rhomboides, Anchoa mitchilli, Leiostomus xanthurus, Membras martinica, Micropogonias undulatus, Gobiosoma robustum, Hyporhamphus unifasciatus and Eucinostomus argenteus (Barba et al. 1991, in Castañeda and Contreras 2001; Gómez and Contreras 1991). The waterfowl is composed of a total of 11,430 individuals and 27 species, of which six are permanent residents, 19 are overwintering species and two are transient migratory species.